A data communication network for processing data on line with communication systems combined with data processing systems was popularized in the art with the popularization of electronic computers and the development of digital signal processing techniques. With respect to small scale communication systems, such as localized communications systems for use within public offices or companies, it is known to use a packet type communication system using communication cables such as coaxial cables which are economical and reliable and exhibit high transmission efficiency.
In the packet type communication system, a bidirectional communication cable is installed in a laboratory or the like, and a number of stations (or dedicated stations) are connected to the cable. The stations transmit messages which are divided into data blocks of, for example, 1000 to 2000 bits. A header including a destination, a communication number, etc. is added to the message. In the communication system, the channel is a passive transmission medium which has no control function. The stations themselves possess all control functions. Accordingly, each station accesses a channel after confirming that the transmission path is empty, to thereby start message transmission. When packets collide with each other during transmission, the two stations suspend the transmission. The stations start the transmission of the messages again after waiting optionally selected periods of time.
In such a communication system, packets collide with each other on the same transmission path because the stations randomly start message transmissions. Accordingly, the communication system suffers from the problem that the transmission delay time is not constant. Thus, the communication system is not suitable for real time transmission in which, as in voice communication, the relation between signal transmission and signal reception is important. This difficulty may be eliminated by employing a method in which a master station is permanently established with dedicated stations making appointments for channel access. However, this method involves the following problem. Data communication is stopped whenever the master station is out of order. Therefore, the communication system is not reliable.
In order to eliminate the above-described drawback, a digital signal transmission system called "Modified Ethernet" has been proposed. In this system, a periodically repeated frame is time divided into a plurality of blocks with the dedicated stations transmitting their information packets within the time blocks. Accordingly, the dedicated stations have equal access to empty blocks. In the case where a station possesses certain blocks for a period of time required for signal transmission, the station may periodically transmit its signals each time the frame, which includes its blocks, is repeated. That is, the real time transmission can be carried out.
FIG. 1 shows a signal frame arrangement in the modified ethernet. The frame which is repeated periodically in time consists of N blocks #1 through #N. Each block consists of various bit trains b.sub.1 through b.sub.9 as listed below:
b.sub.1 : rear guard time PA1 b.sub.2 : preamble PA1 b.sub.3 : address bit PA1 b.sub.4 : distance code bit PA1 b.sub.5 : control bit PA1 b.sub.6 : data bit PA1 b.sub.7 : check bit PA1 b.sub.8 : end flag PA1 b.sub.9 : front guard time
The bit trains b.sub.2 through b.sub.5, b.sub.7 and b.sub.8 are required for forming a packet and are called "overhead (additional) bits." The two bit trains b.sub.1 and b.sub.9 together are called "a guard time." That is, the guard time, represented by empty bit trains is provided to prevent partially overlapped of adjacent packets. There is a tendency for overlapping owing to the delay which results when bit trains are transmitted over a coaxial line. The empty bit trains are the rear guard time b.sub.1 which is positioned after a packet and the front guard time b.sub.9 which is positioned before a packet.
In the above-described arrangement, one block, b.sub.6 forms one packet. However, there may be cases where a plurality of continuous blocks form one packet. In such a case, the packet is still arranged as just described. Such a packet can transmit a lot of data efficiently. Furthermore, the frequency of packet collisions can be reduced as compared to the case where data are transmitted with a plurality of packets.
FIG. 2 outlines a modified ethernet communication system employing the above-described frame arrangement. In this communication system, a coaxial cable 1 connected as a transmission line is, at both ends, connected to impedance matching terminators 2 whose resistance is equal to the coaxial cable's characteristic impendance. Various stations are connected through T-connectors 3.sub.1 through 3.sub.N to the coaxial 1. These stations are fundamentally all the same. Therefore, FIG. 2 shows the essential components of only station A connected to the T-connector 3.sub.1. Each of the other stations have like components.
Each station has a subscriber device 4 including a computer and a telephone. More specifically, the subscriber device 4 comprises a transmitter (encoder) for transmitting digital signals in packets to other stations, a receiver (decoder) for receiving digital signals in packets, and a terminal controller 43 for controlling the terminal. The output signal of the transmitter 41 is temporarily stored in a transmitting buffer memory 51 and is then read out at a predetermined time instant with a clock signal which is equal to the speed of transmission on the coaxial cable 1. The signal thus read out is converted into a predetermined packet in a transmitting logical circuit 52. The packet signal thus formed is applied through a buffer amplifier 53 and the T-connector 3.sub.1 to the coaxial cable 1.
All packet signals transmitted over the coaxial line 1, regardless of the orginating station, are received through the T-connector 3.sub.1 by a receiving buffer amplifer 54. Out of the packets thus received, the one which is destined for that station is selected by a receiving logical circuit 55 and temporarily stored in a receiving buffer memory 56. The signal thus stored is continuously read out with a predetermined clock signal. Thus, a receiving output signal has been obtained.
The transmission and reception of signals are carried out as described above. A transmission clock signal in this operation is generated by a transmission clock generator 57. A frame counter 58 frequency-divides the transmission clock signal, to form a frame timing signal 59 and a block timing signal which specify frame timing and clock timing, respectively. A transmission control circuit 61 controls the terminal controller 43 with the aid of a receiving signal from the receiving logical circuit 55, which is provided for its own station, and further controls the transmitting logical circuit 52 according to an instruction from the terminal controller 43. A collision detection circuit 62 operates to detect when a packet signal is transmitted in a block selected by that station, whether or not the packet signal collides with packets signal from other stations. The subscriber device 4 in each station is provided with a memory (not shown) for indicating the exclusive station assigned to each block in a frame, so that blocks are registered according to the packet signals of the stations which are received by the receiving buffer amplifier 54.
In the modified ethernet communication system, frame synchronization should be established in the stations. For the frame synchronization, one of the stations which is transmitting signals is designated as the leader, and will be referred to as "a master station." The master station transmits a packet signal according to the frame timing signal 59 and the block timing signal 60 which are outputted by the frame counter 58 in the master station. This packet signal is received through the coaxial cable 1 by all the stations. Upon reception of the packet signal, each station resets its own frame counter 58 with predetermined timing. Thus, the frame synchronization has been established for all the stations. In the stations other than the master station, a block timing signal is obtained from the frame counter 58 which is periodically reset, so that the block synchronization is established. The block which the master station used for transmission of the packet signal will be referred to as "a master block." When the master station is going to stop the transmission of the packet signal at a time instant, the master block may not exist in one frame. In order to avoid this difficulty, a new master station is selected in advance. The fact that the master station is replaced as described above will be referred to as "transition of the master station."
In the communication system, a station which is about to transmit a signal searches the established frame for an empty block and loads the packet signal in the block. This will be described in more detail. FIG. 3 shows the arrangement of blocks which belong to two frames. For convenience in description, it is assumed that ten blocks #1 through #10 form one frame. Further, it is assumed that data is transmitted within the fifth block #5 in the n-th frame F(n) (n being an integer) and at this time instant a station makes a request for transmission. This station uses the memory in the subscriber device, which indicates the above-described block arrangement. If this station intends to transmit a packet signal of one block, then it can select as an empty block one of the sixth, eighth and tenth blocks #6, #8 and #10. On the other hand, if the station is to transmit a packet signal 63 of three blocks as shown in FIG. 4, continuous empty blocks are not available for the packet signal, and accordingly the packet signal 63 cannot be transmitted with the n-th frame F(n). In many cases, the block arrangement is not greatly changed even in the next frame F (n+1). That is, if only two contiguous empty blocks are available as shown in FIG. 3, then the packet signal 63 cannot be transmitted with this block.
As is apparent from the above description, in the conventional digital signal transmission system, the stations hold the blocks at random, and sometimes it takes a relatively long period of time until a packet signal over a plurality of blocks is transmitted. That is, the conventional digital signal transmission system suffers from the problem that the average waiting time (or transmission delay times) from the time instant that transmission of signals is requested until the time instant that the transmission is started becomes extensive as the length of a packet signal increases.